[R] Double FOR

[hadley wickham]

> I want to run through a formula several times with several different
> variables (which are defined by independent vectors of equal length 10
> elements). It looks like this:

One way would be explicitly create your data first:
df <- expand.grid(a = 1:5, b= 1:5, c=1:5)

And then take advantage of vectorisation to compute DIC.hat:
with(df, sum(-2*((log(1/sqrt(2*pi*a^2))*exp((((b-c)/a)^2)/-2))))

which looks awfully like differences of two normal densities, so you
might be able to use dnorm (which might be faster as it is written
purely in C (but not noticeably so unless you have a lot of data), but
will make your algorithm more clear).

By breaking it down into multiple steps, hopefully you can get a
better idea of what's going on.  Putting browser() in the middle of
your loop would be another way for you to check out what is really
happening.

Hadley

On 11/26/05, Ginestet, Cedric <c.ginestet at imperial.ac.uk> wrote:
> Hi,
>
>
>
>
>
>
>
>
> function (m,s,y)
>
> {
>
> for (j in m){
>
> for (k in s){
>
> for (i in y){
>
>
>
> DIC.hat<-sum(-2*((log(1/sqrt(2*pi*s^2))*exp((((y-m[j])/s)^2)/-2))))
>
> }
>
> }
>
> }
>
> DIC.hat
>
> }
>
>
>
>
>
> My problem is that R runs the three variables at the same time providing
> me with 10 new elements for DIC.hat, when I would like to have 20 times
> more.
>
>
>
> Can you help?
>
>
>
>
>
> ----------------------------------------------------
>
> Cedric Ginestet
>
> Department of Epidemiology and Public Health
>
> Faculty of Medicine
>
> Imperial College
>
> Norfolk Place
>
> London
>
> W2 1PG
>
> UK
>
> Tel:  +44 (0)77 8688 4313
>
> Fax: +44 (0)20 7402 2150
>
>
>
>
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